1. Field of the Invention
The invention is directed to an optical fiber amplifier having a switchable or interchangeable fiber module for varying the effective length of an amplification fiber.
2. Description of the Related Art
Optical wavelength-division multiplex (WDM) transmission systems must be designed such that they can be operated on links having different amplifier spacings (physical). The problem that arises is illustrated as follows. Line attenuations between 33 dB and 14 dB are provided between the individual, multi-stage amplifiers. This results in the input powers per channel at the amplifier input lying between approximately xe2x88x9228 dBm and xe2x88x9213 dBm. The following presumes that the optical amplifier (inline amplifier) can be divided into a pre-amplifier followed by an attenuation element and a booster (power amplifier). Fixed levels (for example, 0 dBm/channel) at a photodiode are required for optimum detection, so that the power per channel at the output of the amplifier and, thus, at the input of the booster must be constant, i.e., independent of the gain in the individual amplifier sections.
In the above applied example, the gain of the amplifier, which is defined by the length of the doped fiber as well as by the average occupancy inversion in addition to being defined by the fiber parameters, must be capable of being varied between 13 dB and 28 dB. The variation of the amplifier gain is particularly easy via the pump power. When the pump power is reduced, the average occupancy inversion (and, thus, the gain as well) decrease. This, however, is unsuitable for WDM systems since the curve of the gain over the wavelength is highly dependent on the average occupancy inversion {overscore (N)}2. The average occupancy inversion {overscore (N)}2 is defined as the normed average of the occupancy inversions N2i (for all ions with meta-stable levels) of all amplifier stages Vi (for example, 0 less than i less than 3 for a pre-amplifier and a booster) over the entire length of the optical amplifier. The N2i can be functions of the location. Amplifiers for WDM systems must exhibit a flat gain curve that can be additionally realized with the assistance of specific filters in a defined an operating condition. When the average occupancy inversion is modified, then the individual wavelength channels experience a clearly different gain.
A flat gain curve can even be achieved without specific filters in the L-band (approximately 1570-1610 nm), in that an average occupancy inversion of approximately 35% (or approximately {overscore (N)}2=0.35) is set. This percentage is dependent on the erbium-doped fibers employed. Thus, for a gain of 30 dB, the gain differences amount to, e.g., only 1.8 dB. Noticeably greater gain differences occur, however, as soon as the average occupancy inversion changes. A pre-amplifier for WDM systems is therefore usually dimensioned such that it comprises a maximum required, constant gain. By inserting an additional attenuation, the gain can then be reduced to the value needed in a specific application. However, high noise coefficients arise given high attenuations for low gains.
The International Patent document PCT/WO98/36513, xe2x80x9cOptical Fiber Amplifier Having Variable Gainxe2x80x9d, discloses an optical fiber amplifier with a gain control for WDM signal transmission. The circuit is explained in FIG. 2 of this document, in which a controlled attenuation element 5 is inserted between the two amplifier stages 3 and 11. Three photoelectric elements 13, 17 and 25 measure the light power along the amplifier, and regulate the attenuation via a controller. The goal of this regulation is to obtain a variable output gain of the amplifier given a constant curve of all output channel levels of a WDM input signal. Expediently, the xe2x80x9ctiltxe2x80x9d of the gain following the first pre-amplifier stage is compensated by a spectral gain smoothing filter 9. A spectrally uniform gain can thus be achieved at the output of the amplifier for all WDM channels. An attempt is made in the general case to keep the gain and, thus, the noise of the first pre-amplifier stage low. The attenuation is boosted for high input levels. However, this still produces high noise coefficients.
The following paragraphs discuss a few versions optical amplification according to the Prior Art, illustrated in FIGS. 1-4. Their properties are explained and the disadvantages that are eliminated by the present invention are described.
As shown in FIG. 1, the amplifier is divided into two stages V1 and V2 between which a variable attenuation element DG is inserted. FIG. 1 also shows the gain G and the signal power P (or level) along the fiber amplifiers for two different amplifications.
For low input levels of the WDM signal S, an amplification ensues with a high gain GG. The broken line refers to the high gain GG of a signal with low input power.
For high input levels, a low gain KG given a poor noise coefficient is achieved due to the high attenuation between the amplifier stages. The solid line refers to the low gain KG of a signal with high input power. The high levels are highly attenuated by the attenuation element DG between the two amplifier stages V1 and V2.
The amplifier in FIG. 2 comprises the same components V1, V2 and DG as in FIG. 1, but the attenuation element DG is attached directly to the output of the second amplifier stage V2. This version enables a variable amplification with good noise coefficients for low and high gains G. The levels in the amplifier, however, will reach high values for signals with a high input power, resulting in non-linearities occurring particularly in the L-band for high levels in the amplifier stage V2. However, the average occupancy inversion {overscore (N)}2 must be maintained given increased input power, requiring clearly higher pump powers. It can be clearly seen in FIG. 2 that signals having high input power are amplified unnecessarily with high pump power and are in turn attenuated at the output. This version, however, solves the previously identified noise coefficient problems of the amplifier according to FIG. 1.
FIG. 3 shows the typical structure of an L-band amplifier. The occupancy inversion is schematically shown as function of the fiber length L. It is critical that the occupancy inversion N21 in the first section S1 of the erbium-doped fibers EDF is very high in order to keep the noise coefficient low. The desired, average occupancy inversion {overscore (N)}2 is then set with the following sections of lower occupancy inversion. The average occupancy inversion {overscore (N)}2 is calculated as                     N        _            2        =                  1                  L          G                    ⁢                        ∑          i                ⁢                  xe2x80x83                ⁢                              ∫                          L              i                                ⁢                                                    N                                  2                  ⁢                  i                                            ⁡                              (                l                )                                      ⁢                          xe2x80x83                        ⁢                          ⅆ              l                                            ,
where LG references the overall length of the erbium-doped fibers in the i various amplifier stages. N2i references the occupancy inversion of the ith amplifier stage with fiber length Li. Only two amplifier stages with corresponding occupancy inversions N21, and N22 are shown in this example. The occupancy inversions N22 should be as small as possible, so that the first fiber section becomes optimally long with a given, average occupancy inversion.
FIG. 4 shows a fiber amplifier that is adaptable to different input levels. A respective, doped fiber section EDFE and EDFA outside a module housing MG is located at the input and output. The gain is then varied in that these fiber sections are replaced by correspondingly shorter or longer ones. For a change of the lengths of the two fiber sections by xcex94L1 and xcex94L2, where N21, and N22 are kept constant and xcex94L1xc3x97L2=xcex94L2xc3x97L1 is also valid, the average occupancy inversion {overscore (N)}2 does not change and the relative gain differences continue to be slight. The outlay given this version is too great since an extensive set of fibers is needed for all required gain values.
Therefore, only a supply of a few fibers and, further, an attenuation element DG are provided. The configuration that minimally exceeds the desired gain is sought from the supply of fibers. The fine adjustment then occurs with the attenuation element DG. Since the insertion attenuations are now clearly lower, the auxiliary attenuation has less of an influence on the noise coefficient. One advantage of this arrangement is that the occupancy inversions N21, and N22 need not be adapted. It is disadvantageous that two fiber sections must be replaced.
The object of the present invention is to provide a simple mechanism for adapting the fiber amplifier in WDM systems to different input levels or gain values for optimum noise behavior.
This object is inventively achieved by an optical fiber amplifier comprising a switchable or interchangeable fiber module for varying an effective length of an amplification fiber. The fiber module may further comprises a connecting fiber, an amplification fiber, and/or an attenuation element. The fiber module may comprise an amplification fiber divided into two amplification fiber sections with an intervening attenuation element. The optical fiber amplifier may also comprise a first amplifier stage; and a last amplifier stage, the fiber module being arranged between said first amplifier stage and said last amplifier stage. The fiber module may further comprise an attenuation element, and a maximally set attenuation of said attenuation element is less than the difference between a maximum and a minimum amplifier gain. The optical fiber amplifier may further comprise filters in an amplifier module or in said fiber module for leveling the spectrum of signal levels. The fiber module may further comprise a dispersion-compensating fiber, potentially with a pump source, and/or a separate gain control for the dispersion-compensating fiber.
The inventive amplifier allows an expansion of the range of amplification without having to highly attenuate the signal following the pre-amplification. This results in the noise coefficients remaining small at the different amplification values, particularly given low amplifications as well.
The gain in the inventive amplifier is directly set by different fiber lengths of a module. Only one interchangeable or switchable, passive fiber module is provided. The great advantage of the illustrated solution is that the auxiliary module FM is purely passive and, thus, no further terminals other than the optical input and output are required.
Only one auxiliary module is usually required for setting two different amplification values; the fine adjustment of the amplification value is realized with an attenuation element having a low attenuation value.
Given a correct setting of the pump powers, the average occupancy inversion {overscore (N)}2 is kept such given variation of the fiber length that the amplifier exhibits a flat gain spectrum for the amplification of all WDM channels.
In the inventive amplifier, the remaining amplification differences can be minimized by a specific, additional filter suitable for different gains.
The use of a plurality of fiber modules that are optimized for achieving different amplifications is also possible. One auxiliary module can basically be utilized in each amplifier stage. A plurality of modules can also be utilized in different amplifier stages. The best configuration is selected on the basis of the specific demands.
The switching between these fiber modules can be done with plug-type connectors or with switches. In practice, however, the use of a fiber amplifier without or with an additionally inserted fiber module suffices for achieving the required amplification values from 20 db through 30 dB.